Carbon steel sheet having excellent carburization properties, and method for producing same

ABSTRACT

The invention provides a carbon steel sheet including C: 0.20% to 0.45% by mass, Si: 0.05% to 0.8% by mass, Mn: 0.85% to 2.0% by mass, P: 0.001% to 0.04% by mass, S: 0.0001% to 0.006% by mass, Al: 0.01% to 0.1% by mass, Ti: 0.005% to 0.3% by mass, B: 0.0005% to 0.01% by mass and N: 0.001% to 0.01% by mass, in which a K value that can be obtained from 3C+Mn+0.5Si is greater than or equal to 2.0; surface hardness is less than or equal to 77 on the Rockwell B Scale; and the average content of N in a zone from the surface to a depth of 100 μm is less than or equal to 100 ppm. This carbon steel sheet is configured to be carburized in a carburization atmosphere with a carbon potential of 0.6 or less.

TECHNICAL FIELD

The present invention relates to a carbon steel sheet having excellent carburization properties and a method for producing the same.

The present application claims priority based on Japanese Patent Application No. 2009-079959 filed in Japan on Mar. 27, 2009, and its content is incorporated by reference herein.

BACKGROUND ART

In the past, industrial machine parts or automobile parts, such as chains, gears, or clutches, were produced by hardening the surfaces thereof with a thermal treatment, such as quenching, after a forming process.

However, in recent years, as the forms of parts have become more complex, abrasion resistance, fatigue characteristics, or the like have been in demand. Therefore, materials are required to satisfy not only workability that can withstand complicated processes while processing parts but also hardenability for surface hardening. The hardenability and workability of materials are opposing characteristics in terms of material design. In general, softening of materials is effective for the improvement of workability, but most of elements for enhancing hardenability increase the hardness of the steel sheet, and thus sacrifice workability.

On the other hand, if hardenability of parts after the processing is not adequate, abnormal layers in which structures such as perlite, sorbite or troostite are mixed are generated in the products.

In order to manufacture steel sheets having excellent workability and hardenability at a low cost, it is effective to add B in the steel sheets. However, due to the reactivity of B, changes, such as oxidation, deboronization, or nitrogenization, occur at the surfaces of the steel sheet, therefore it is difficult to secure hardenability at a surface layer portion.

In addition, in steel sheets in which B has been added (hereafter referred to as B-added steel sheets) and carburizing has been performed at a carbon potential (Cp) of about 0.8 (which is commonly used), carburized carbon increases hardenability and thus it becomes difficult for quenched abnormal layers to occur at a surface layer portion after quenching, therefore no serious problem occurs. However, in weakly carburized zones with a low carbon potential (for example, Cp≦0.6), B degrades hardenability due to the above reaction, and furthermore, hardenability by carbon (C) also cannot be secured, therefore the B-added steel sheets are not widely used.

The carbon potential mentioned herein refers to a value indicating the carburizing capability of atmospheres when carburizing steel materials. The carbon potential is equivalent to the carbon concentration at steel surfaces when reaching an equilibrium with a gaseous atmosphere at a carburizing temperature.

Therefore, in B-added steel sheets, material optimization is required throughout all the processes from materials to parts processing, such as the establishment of production conditions in which the effects of the addition of B can be sufficiently obtained and the securement of workability for severe processes, such as profile forming, and treatability of surface hardening, such as carburizing.

Regarding production conditions of B-added steel sheets, Patent Document 1 discloses annealing under a hydrogen atmosphere with nitrogen content suppressed to 10% or less by volume or an argon atmosphere, but nothing is described regarding the prior or subsequent processes. In addition, there is no technology disclosed regarding a carburizing treatment at a low carbon potential which is the subject of the invention.

RELATED ART DOCUMENT Patent Document

-   [Patent Document 1] Japanese Unexamined Patent Application, First     Publication No. H5-331534

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

An object of the invention is to provide a B-added steel sheet having excellent hardenability even in a carburization with a low carbon potential condition and furthermore equipped with workability, and to optimize a method for producing thereof, in order to solve the above-described problems.

Means for Solving the Problems

The invention adopts the following measures to solve the above described problems.

(1) A first aspect of the invention is a carbon steel sheet configured to be carburized in a carburization atmosphere with a carbon potential of 0.6 or less, including: C, 0.20% to 0.45% by mass, Si: 0.05% to 0.8% by mass, Mn: 0.85% to 2.0% by mass, P: 0.001% to 0.04% by mass, S: 0.0001% to 0.006% by mass, Al: 0.01% to 0.1% by mass, Ti: 0.005% to 0.3% by mass, B: 0.0005% to 0.01% by mass and N: 0.001% to 0.01% by mass with a balance including Fe and inevitable impurities, wherein K value that can be obtained from 3C+Mn+0.5Si is greater than or equal to 2.0; surface hardness is less than or equal to 77 on Rockwell B Scale; and an average content of N in a zone from a surface to a depth of 100 μm is less than or equal to 100 ppm.

(2) The carbon steel sheet in the above (1) may further include one or more components selected from Nb: 0.01% to 0.5% by mass, V: 0.01% to 0.5% by mass, Ta: 0.01% to 0.5% by mass, W: 0.01% to 0.5% by mass, Sn: 0.003% to 0.03% by mass, Sb: 0.003% to 0.03% by mass, and As: 0.003% to 0.03% by mass.

(3) A second aspect of the invention is a method for producing the carbon steel sheet according to claim 1 or 2, including: heating a slab to less than or equal to 1200° C.; hot-rolling the slab at a final rolling temperature of 800° C. to 940° C. so as to obtain a steel sheet; cooling the steel sheet at a cooling rate of 20° C./second or more until a temperature of the steel sheet becomes less than or equal to 650° C., as a first cooling; cooling the steel sheet at a cooling rate of 20° C./second or less, as a second cooling subsequent to the first cooling; coiling the steel sheet at a temperature of 400° C. to 650° C.; pickling the steel sheet; and annealing the steel sheet for 10 hours or more at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C., as a first annealing.

(4) In the method for producing the carbon steel sheet described in the above (3), the first annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the first annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(5) The method for producing the carbon steel sheet described in the above (4) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the pickling, as a first cold-rolling.

(6) The method for producing the carbon steel sheet described in the above (5) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.

(7) In the method for producing the carbon steel sheet described in the above (6), the second annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(8) The method for producing the carbon steel sheet described in the above (7) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

(9) In the method for producing the carbon steel sheet described in the above (8), the third annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(10) The method for producing the carbon steel sheet described in the above (6) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

(11) In the method for producing the carbon steel sheet described in the above (10), the third annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(12) The method for producing the carbon steel sheet described in the above (4) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.

(13) In the method for producing the carbon steel sheet described in the above (12), the second annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(14) The method for producing the carbon steel sheet described in the above (13) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

(15) In the method for producing the carbon steel sheet described in the above (14), the third annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Act1+50° C., and after the third annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(16) The method for producing the carbon steel sheet described in the above (12) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

(17) In the method for producing the carbon steel sheet described in the above (16), the third annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(18) The method for producing the carbon steel sheet described in the above (3) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the pickling, as a first cold-rolling.

(19) The method for producing the carbon steel sheet described in the above (18) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.

(20) In the method for producing the carbon steel sheet described in the above (19), the second annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(21) The method for producing the carbon steel sheet described in the above (20) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

(22) In the method for producing the carbon steel sheet described in the above (21), the third annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Act1+50° C., and after the third annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(23) The method for producing the carbon steel sheet described in the above (19) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

(24) In the method for producing the carbon steel sheet described in the above (23), the third annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(25) The method for producing the carbon steel sheet described in the above (3) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.

(26) In the method for producing the carbon steel sheet described in the above (25), the second annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(27) The method for producing the carbon steel sheet described in the above (26) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

(28) In the method for producing the carbon steel sheet described in the above (27), the third annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(29) The method for producing the carbon steel sheet described in the above (25) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

(30) In the method for producing the carbon steel sheet described in the above (29), the third annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(31) A third aspect of the invention is a′ carbon steel sheet configured to be carburized in a carburization atmosphere with a carbon potential of 0.6 or less, including C, 0.20% to 0.45% by mass, Si: 0.05% to 0.8% by mass, Mn: 0.85% to 2.0% by mass, P: 0.001% to 0.04% by mass, 0.0001% to 0.006% by mass, Al: 0.01% to 0.1% by mass, Ti: 0.005% to 0.3% by mass, B: 0.0005% to 0.01% by mass and N: 0.001% to 0.01% by mass, and further including one or more components selected from: Cr: 0.01% to 2.0% by mass, Ni: 0.01% to 1.0% by mass, Cu: 0.005% to 0.5% by mass and Mo: 0.01% to 1.0% by mass with a balance including Fe and inevitable impurities, wherein K′ value that can be obtained from 3C+Mn+0.5Si+Cr+Ni+Mo+Cu is greater than or equal to 2.0; surface hardness is less than or equal to 77 on Rockwell B Scale; and an average content of N in a zone from a surface to a depth of 100 μm is less than or equal to 100 ppm.

(32) The carbon steel sheet in the above (31) may further include one or more components selected from Nb: 0.01% to 0.5% by mass, V: 0.01% to 0.5% by mass, Ta: 0.01% to 0.5% by mass, W: 0.01% to 0.5% by mass, Sn: 0.003% to 0.03% by mass, Sb: 0.003% to 0.03% by mass, and As: 0.003% to 0.03% by mass.

(33) The carbon steel sheet described in the above (31) or (32) is the method for producing the carbon steel sheet described in the above (31) or (32) including: heating a slab to less than or equal to 1200° C.; hot-rolling the slab at a final rolling temperature of 800° C. to 940° C. so as to obtain a steel sheet; cooling the steel sheet at a cooling rate of 20° C./second or more until a temperature of the steel sheet becomes less than or equal to 650° C., as a first cooling; cooling the steel sheet at a cooling rate of less than or equal to 20° C./second, as a second cooling subsequent to the first cooling; coiling the steel sheet at a temperature of 400° C. to 650° C.; pickling the steel sheet; and annealing the steel sheet for more than or equal to 10 hours at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C., as a first annealing.

Effects of the Invention

In the configurations described in the above (1) and (31), since it is defined that the K value and the K′ value are greater than or equal to 2.0, and the average amount of N in surface layers is less than or equal to 100 ppm, it is possible to develop high hardenability even in a carburization with a low carbon potential condition and thus obtain a B-added carbon steel sheet equipped with high workability.

According to the configurations described in the above (2) and (32), it is possible to obtain an effect of stabilizing precipitates or improving toughness or an effect of suppressing component variations in the surface layer portion of a steel sheet.

According to the methods described in the above (3) and (33), it is possible to stably produce a carbon steel sheet having excellent workability and post-processing carburization treatability.

According to the methods described in the above (4) to (30), it is possible to further improve the workability or softening of a carbon steel sheet.

As described above, according to the invention, it is possible to produce a steel material having not only excellent carburization properties so as to prevent the generation of abnormal layers due to inferior hardenability while carburizing a B-added steel but also excellent workability for producing parts or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a relationship between the K value or the K′ value and the average amount of N in a surface layer relating to the generation of abnormal layers during carburization.

FIG. 2 is a diagram showing a relationship between a crack in the profile formed portions and material hardness during the profile forming process.

FIG. 3 is a flowchart to explain the production method.

EMBODIMENTS OF THE INVENTION

The inventors conducted hardness variation or structure investigation at a surface layer portion during carburized quenching with a variety of changes in the components of B-added steel sheets or production conditions during production processes, and clarified the relationship between the structures and components of the surface layers which affect hardenability of the surface layers. As a result, it was found that there were cases in which, instead of martensite, structures which are more softened than martensite, such as pearlite, sorbite, troostite, were generated, and, particularly, such structures were often observed in the outermost surface layers from the surface to a depth of about 100 μm.

FIG. 1 shows the generation of abnormal layers in a 0.22% C-based material which has been carburized quenched at a carbon potential of 0.3. It was found that the abnormal layers have a significant relationship with the content of nitrogen (N) present in a surface layer of the steel sheet to a depth of 100 μm (the average amount of N in the surface layer) and the K values (or the K′ values) obtained by steel sheet components.

Here, the average amount of N in the surface layer refers to a value obtained by analyzing the content of nitrogen (N) in shavings taken after planing the surface of the steel sheet to a depth of 100 μm before carburization.

In order to observe the effect of steel sheet components, the K value represented by Formula (1) and the K′ value represented by Formula (2) were introduced.

K value=3C+Mn+0.5Si  (1)

Wherein C, Mn and Si represent the content of each element (% by mass).

K′ value=3C+Mn+0.5Si+Cr+Ni+Mo+Cu  (2)

Wherein C, Mn, Si, Cr, Ni, Mo and Cu represent the content of each element (% by mass). Further, they will be zero if the corresponding elements are not included.

As shown in FIG. 1, it was found that abnormal layers are not observed in a zone where the K value (the K′ value in a case in which Cr, Ni, Mo and Cu are included) is greater than or equal to 2.0 and the average amount of N in the surface layer is less than or equal to 100 ppm so that carburization properties are excellent. The reason why such a favorable zone as above can be obtained is because, as the average amount of N in the surface layer increases, the amount of nitrogen (N) precipitated as nitrides during the production processes is increased, and the growth of autenite grains are delayed during carburization, thereby degrading hardenability. In particular, it is considered that, since B is nitrogenized by N so as to become BN, the amount of solute B in the steel is decreased and thereby, the hardenability of the steel sheet is impaired.

In addition, in terms of steel sheet hardenability, the steel sheet needs to include alloy elements to a certain level, and the hardenability could be clarified by indicating the amount of the alloy elements with the K value (or K′ value) shown above. A higher K value (or K′ value) is advantageous to secure higher hardenability, but, if the K value (or K′ value) is too high, there are cases in which problems occur, such as increased steel sheet hardness that degrades workability, or the occurrence of hardening cracks during quenching depending on the form of parts. The upper limit of the K value (or K′ value in a case in which Cr, Ni, Mo and Cu are included) is not particularly specified, but, if the value exceeds 3.6, hardenability becomes too high, and thus defects, such as the above hardening crack or the like, will occur, therefore the value is desirably less than or equal to 3.6.

In terms of steel sheet workability, the invention defined the surface hardness of the steel sheet as less than or equal to 77 on the Rockwell B Scale (HRB). The steel material according to the invention, which may be used for automobile parts or the like, may subject to severe processes such as tooth-shape forming (profile forming) of gear parts. Therefore, workability that can withstand such severe processes is required.

In the invention, as the evaluation of workability, it was investigated whether a crack was formed in an area in the base portion of a profile formed portion where shear deformation occurs after conducting a processing test which simulated profile forming process. Using a 0.22% C-based steel material, 3 mm-thick steel sheets were produced under varied conditions of hot-rolling, cold-rolling and annealing so as to prepare test specimens. As to the shape of the profile formed object, a rack-shaped die was produced at a module of 1.5 mm defined by JIS-B1703, and 3 mm-thick steel sheets were 2 mm-pressed, and then whether or not cracks occurred in the profile formed portions was evaluated.

FIG. 2 shows the results. Crack occurrence from severe processes, such as profile forming, shows a favorable correspondence with surface hardness, and it was found that it is effective to achieve softening with a surface hardness of HRB 77 or less to produce a material that can withstand profile forming.

Meanwhile, in the invention, in terms of securing hardenability as described above, the lower limit of the K value (or K′ value) is defined. A higher K makes a material harder and thus is advantageous for hardness during quenching, but degrades workability, therefore problems, such as the formation of cracks, occur during processes. As a result, it is necessary to carry out the production method defined in the invention and to carry out softening of steel sheets while controlling the annealing atmosphere.

Hereinafter, steel sheet components and production conditions will be described.

C is a basic element necessary to obtain the strength of a steel sheet. With a carbon content of less than 0.20%, it is not possible to obtain the strength demanded to produce products, and hardenability is also degraded at the core portion of the parts so that desired characteristics cannot be obtained. However, since if a large amount of C exceeding 0.45% is included, it is difficult to secure toughness and formability after thermal treatments, the content of C is specified in a range from 0.20% by mass to 0.45% by mass (hereinafter, unless otherwise described, contents will have a unit of % by mass). A more preferable range is from 0.20% to 0.40%.

Si is used as a deoxidizing agent of steel and is also effective in terms of hardenability. It is necessary to include 0.05% or more of Si. However, since as the content of Si increases, degradation of surface texture occurs due to scale defects or the like during hot-rolling, the upper limit was defined as 0.80%. A more preferable range is 0.05% to 0.50%.

Mn is used as a deoxidizing agent and is also effective in terms of hardenability. In terms of securing hardenability during carburization carried out at a low Cp, addition of 0.85% or more of Mn is required in the invention. An excessive content of Mn results in degradation or scattering (variation) of impact characteristics caused by segregation-induced structural variation after quenching and tempering, therefore the upper limit is defined as 2.0%. A more preferable range is from 0.90% to 1.80%.

In the steel of the invention, P is a harmful element in terms of toughness or workability, therefore a lower content of P is desirable and the upper limit is defined as 0.04%. In addition, the lower limit is desirably lower, but a decrease in the content below 0.001% significantly raises industrial costs, therefore the lower limit is defined as 0.001%. A more preferable range is 0.003% to 0.025%.

S accelerates the generation of non-metallic inclusions in steel so as to degrade forming workability, toughness after thermal treatments, or the like. As a result, a lower content of S is desirable, and the upper limit thereof is defined as 0.006%. The lower limit is desirably lower, but a decrease in the content below 0.0001% significantly raises industrial costs, therefore the lower limit is defined as 0.0001%. A more preferable range is from 0.0001% to 0.003%.

Al is used as a deoxidizing agent of steel, and therefore 0.01% or more of Al is required. However, even when more than 0.10% of Al is added, the effect is saturated, and scale defects are likely to occur. In addition, Al is also effectively bonded with N and accelerates nitrogen absorption during steel sheet production. However, if the content exceeds 0.10%, Al nitrides are stabilized so as to hinder grain growth during carburization thermal treatments and degrade hardenability. As a result, the content of Al is defined in a range from 0.01% to 0.10%. A more preferable range is from 0.01% to 0.06%.

Ti is effective as a deoxidizing agent of steel. In addition, Ti effectively bonds with N. Therefore, it is necessary to add 0.005% or more of Ti from the relationship with the amount of N. However, even when more than 0.30% of Ti is added, the effect is saturated, and the cost also rises. Furthermore, since the amount of precipitates induced by nitrogen absorption during production processes is increased, grain growth is hindered during carburization and hardenability is degraded. As a result, the content of Ti is defined in a range from 0.01% to 0.30%. A more preferable range is 0.01% to 0.10%.

B is an effective element to improve the hardenability of steel, and such an effect can be achieved with an extremely small amount. In order to obtain the effect of hardenability improvement, it is necessary to add 0.0005% or more of B. However, if a large amount of B exceeding 0.01% is included, castability is degraded and cracks occur during slab casting. Furthermore, B-based compounds are generated in steel so as to cause adverse effects, such as a decrease in toughness. As a result, the content of B is defined in a range from 0.0005% to 0.01%. A more preferable range is 0.0005% to 0.005%.

N is bonded with B so as to generate nitrides and degrades the hardenability improvement effect of B. Therefore, a lower content of N is preferable, but a decrease in the content below 0.001% leads to an increase in costs. In addition, if the content of N exceeds 0.01% as an average composition of steel, a large amount of elements that bond with N, such as Al or Ti, are required, and precipitates, such as AlN or TiN, hinder grain growth during carburization so as to degrade hardenability, which not only results in generation of abnormal layers but also degrades mechanical characteristics, such as toughness. As a result, the upper limit of N content is defined as 0.01%. A more preferable range is 0.001% to 0.006%.

In addition, N is likely to intrude into steel during production processes and is introduced from the atmosphere during hot-rolling and heating or annealing, and, in particular, is likely to be concentrated in the surface layer portion, therefore it is necessary to suppress such effects in order to prevent the degradation of hardenability of parts in the surface layer portion. If the amount of nitrogen intruded from atmosphere during heating or annealing exceeds 100 ppm, the amount of precipitated nitrides becomes large during coiling or annealing, and grain growth is delayed during heating before quenching, thereby degrading hardenability. As a result, it is important to define the content of N particularly in the surface layer portion (a zone from the surface to a depth of 100 μm) (the average amount of N in the surface layer) as less than or equal to 100 ppm. The amount of N in the surface layer portion is further preferably less than or equal to 70 ppm.

Cr is an effective element that can be added in terms of the hardenability of steel, and the effect becomes remarkable with a content of 0.01% or more, but even when more than 2% of Cr is added, the effect is saturated, and the cost also rises. As a result, the content is defined in a range from 0.01% to 2.0%. A more preferable range is 0.05% to 0.50%.

Ni is an effective element in terms of improvement in the hardenability or toughness of steel, and addition of 0.01% or more is effective, but addition of more than 1% of Ni merely results in an increase in costs and rarely changes the effect, therefore the content is defined in a range from 0.02% to 1.0%. A more preferable range is 0.05% to 0.50%.

Cu is an effective element in terms of improvement in the hardenability or toughness of steels, and addition of 0.01% or more is effective, but addition of more than 0.5% of Cu merely results in an increase in costs and rarely changes the effect, therefore the content is defined in a range from 0.005% to 0.5%. A more preferable range is 0.02% to 0.35%.

Mo is an effective element that improves the hardenability of steel and an effective element to increase resistance against softening by tempering. In order to obtain such effects, addition of 0.01% or more is required. However, even when more than 1.0% of Mo is included, the effect is saturated, and the cost also rises, therefore the content is defined in a range from 0.01% to 1.0%. A more preferable range is 0.01% to 0.40%.

0.01% or more of Nb has effects of forming carbonitrides, stabilizing precipitates or improving toughness, but addition of more than 0.5% of Nb merely results in an increase in costs and a decrease in hardenability by the formation of carbides, therefore the content is defined in a range from 0.01% to 0.5%. A more preferable range is 0.01% to 0.20%.

Similarly to Nb, 0.01% or more of V has effects of forming carbonitrides, stabilizing precipitates or improving toughness, but addition of more than 0.5% of V merely results in an increase in costs and rarely changes the effect, and also lowers hardenability by the formation of carbides. Therefore, the content is defined in a range from 0.01% to 0.5%. A more preferable range is 0.01% to 0.20%.

Similarly to Nb and V, 0.01% or more of Ta has effects of forming carbonitrides, stabilizing precipitates or improving toughness, but addition of more than 0.5% of Ta merely results in an increase in costs and rarely changes the effect, and also lowers hardenability by the formation of carbides. Therefore, the content is defined in a range from 0.01% to 0.5%. A more preferable range is 0.01% to 0.30%.

Similarly to Nb, V and Ta, 0.01% or more of W has effects of forming carbonitrides, stabilizing precipitates or improving toughness, but addition of more than 0.5% of W merely results in an increase in costs and rarely changes the effect, and also lowers hardenability by the formation of carbides. Therefore, the content is defined in a range from 0.01% to 0.5%. A more preferable range is 0.01% to 0.20%.

Furthermore, in addition to the above, in the invention, in order to suppress component variation in the surface layer portion of steel sheets, a certain amount of one or more components selected from Sn, Sb and As may be added.

Sn, Sb, As: 0.003% to 0.03%

Sn, Sb and As are elements having a high tendency of segregating at interfaces, surfaces or the like and a function that suppresses surface reaction, such as nitrogen absorption or decarburization, during production processes. Therefore, the addition thereof has an effect of preventing remarkable component variation by suppressing the reaction of elements which are liable to induce component variation, such as nitrogen or carbon, even in a state in which steel materials are exposed to a high-temperature atmosphere during heating or annealing in a hot-rolling process. Therefore, Sn, Sb and As may be optionally added. With regard to the each added amount, if the amount is less than 0.003%, the effect is small, and, addition of a large amount exceeding 0.03% not only saturates the effect but also results in a decrease in toughness and an increase in costs by extending carburization time. As a result, it is desirable to be added in a range from 0.003% to 0.03%.

In the steel sheet according to the invention, the content of oxygen (O) is not defined, but, if oxides are agglomerated and thus coarsened, ductility is lowered, therefore the content of oxygen is preferably less than or equal to 0.025%. A lower content of oxygen is preferable, but a content of less than 0.0001% is technically difficult to achieve, therefore the content is preferably more than or equal to 0.0001%.

In addition, the carbon steel sheet according to the invention may include impurities inevitably mixed during production processes in addition to the above elements, but it is preferable to prevent impurities from being mixed therein as much as possible.

Next, production conditions will be described with reference to the flowchart in FIG. 3.

In the invention considering the consistent optimization of steel material components and an annealing process thereafter, hot-rolling is important, and it is important to intensively suppress component variation in the surface layer portion of steel sheets, that is, the intrusion of N into or decarburization in the surface layer portion. Therefore, heating is conducted at 1200° C. or less without applying high-temperature heating which is commonly used and conducted at a temperature of more than 1200° C. (S1). Furthermore, in this case, as soaking time is extended, nitrogen intrusion into the surface layer portion is also increased, and hardening characteristics of the products are affected, therefore it is important not to conduct heating for a long time. Specifically, it is preferable to conduct heating for a retention time not exceeding 60 minutes at 1200° C. and 90 minutes at 1100° C.

Next, hot-rolling is conducted at a final rolling temperature of 800° C. to 940° C. (S2). If the final rolling temperature is lower than 800° C., many burn-in-induced defects occur, and, if the final rolling temperature is higher than 940° C., the generation frequency of scale-induced defects is increased, and thus the product yield ratio is decreased, thereby increasing costs.

After finishing the final hot-rolling, cooling is conducted to 650° C. or less at a cooling rate of 20° C./second or more (S3, first cooling). If the cooling to 650° C. after finishing the rolling is conducted at a rate slower than 20° C./second, structural variations called pearlite bands resulting from segregation occur, which leads to degradation of workability. Therefore, the cooling rate is controlled at 20° C./second or more to a temperature of 650° C. or less after finishing the rolling, and then at 20° C./second or less to a coiling temperature for slow cooling which is supposed to be conducted on homogeneous pearlite transformation, pearlite+bainite structure, bainite structure or the like (S4, second cooling). Thereby, it is possible to suppress the occurrence of structural heterogeneity in the coils. In addition, with respect to the coiling temperature, it is possible to reduce structural variation in the coils by conducting coiling at a temperature of 400° C. to 650° C. which is to achieve structural homogeneity as described above (S5). The hot-rolled steel sheets produced by the above processes are pickled (S6). After pickling, annealing or cold-rolling is conducted as necessary depending on product sheet thickness or necessary levels of softening, but the following is important as production conditions in this case.

With respect to annealing, since the steel sheet according to the invention has a high carbon content, it is not possible to obtain the characteristics by a continuous annealing process that is used for soft steel sheets. Basically, a process in which coils are annealed as they are, such as batch annealing or box annealing, is applied (S7, first annealing).

In this case, in terms of preventing nitrogen concentration in the surface layer portion, an annealing atmosphere majorly includes hydrogen and has a hydrogen concentration of 95% or more. In addition, in the case of performing annealing in a hydrogen atmosphere, in terms of safety, the inside of an annealing furnace is firstly substituted with nitrogen at room temperature so as to form a nitrogen atmosphere, and then substituted with hydrogen. In this case, it is desirable to raise the temperature after substituting with hydrogen in terms of preventing nitrogenization, but the atmosphere may be substituted with hydrogen while raising temperature from a nitrogen atmosphere, and it is necessary to have a hydrogen concentration of 95% or more at a possible low temperature. In addition, in terms of preventing component variation in the surface layer portion, it is important to have, particularly, a dew point of more than or equal to −20° C. up to 400° C. and a dew point of less than or equal to −40° C. during retention at a temperature of more than or equal to 400° C. (retention time depends on materials, but 10 hours or more of retention at a temperature of 660° C. or more is desirable to soften the steel sheet according to the invention), and, if a dew point is high, deboronization, decarburization or the like occurs, and poorly-quenched abnormal layers are generated in a ease of performing carburization at a low carbon potential. By completing the above series of processes (hot-rolling+thermal treatments), the steel sheet according to the invention having excellent workability and, furthermore, excellent carburization properties during a carburization treatment after processing can be obtained.

In terms of softening, high-temperature annealing at a temperature of Ac1 or more is also effective. It is preferable to conduct annealing in a temperature range of “Ac1” to “Ac1+50° C.”, and then set a cooling rate of 5° C./hour so as to cool it to “Ac1−30° C.” after the annealing. Thereby, ferrite phases generated during cooling with a cooling rate of 5° C./hour or less are likely to be coarsened and softening is accelerated by austenite phases generated at Ac1 or more due to scavenging action by the fine carbides. If annealing is conducted at a temperature greater than “Ac1+50° C.”, in the components of the steel according to the invention, the phase ratio of austenite phases becomes too high and pearlite is generated at some places during cooling which hardens the steel, therefore, the temperature of high-temperature annealing in the present invention is preferably less than or equal to “Ac1+50° C.”. In addition, in the steel according to the invention, even when slow cooling is conducted after the temperature reached “Ac1−30° C.”, the effect is saturated and an extended annealing time results in an increase in costs, therefore the end-point temperature of slow cooling is preferably “Ac1−30° C.”.

Here, Ac1 represents a temperature at which austenite phases appear in the temperature-raising process, and, in the invention, A1 transformation points were obtained by taking samples from hot-rolled steel sheets and measuring expansion curves with a Formaster tester when raising the temperature at 0.3° C./second. In addition, written references also disclose simpler methods obtaining Ac1 from components, and an example thereof is Ac1 (° C.)=723−10.7×% Mn−16.9×% Ni+29.1×% Si+16.9×% Cr+290×% As+6.38×% W disclosed in “The Physical metallurgy of Steel” written by William C. Leslie, and such empirical formulae can be used.

Furthermore, the cold-rolling process is used to complete sheet product thickness with a high accuracy and to efficiently conduct softening in combination with annealing. Therefore, in the above series of processes, cold-rolling (S6-2, first cold-rolling) may be conducted after conducting the hot-rolling and coiling (S5) and the pickling (S6). Particularly, by cold-rolling with a rolling ratio of 5% or more, carbides are accelerated to be spherical, and recrystallization not accompanied by nuclei generation or softening in which grain diameters are relatively large when completing recrystallization and grain growth-induced coarsening is likely to occur is accelerated.

The upper limit is not particularly specified, but, if rolling is conducted with a rolling ratio exceeding 60%, homogeneity of the metallic structure of the steel sheet is further increased by cold-rolling, but a higher cold-rolling ratio makes grains recrystallized during annealing smaller, and thus annealing time needs to be extended for softening, therefore, the cold-rolling ratio can be determined in terms of costs and product homogenization.

In the production method according to the invention, it is possible to conduct another cold-rolling with a rolling ratio of 5% or more (S7-2, second cold-rolling) on the steel sheet and then conduct annealing in an atmosphere including 95% or more of hydrogen (S7-3, second annealing) after the above annealing. By undergoing the processes of the cold-rolling (S7-2, second cold-rolling) and an annealing (S7-3, second cold-rolling) after the above annealing (S7-1, first annealing), structural homogenization or crystal grain coarsening can be achieved, and it is possible to further proceed with workability improvement or softening.

In the production method according to the invention, it is possible to conduct additional cold-rolling with a rolling ratio of 5% or more (S7-4, third cold-rolling) on the steel sheet and then conduct annealing in an atmosphere including 95% of hydrogen (S7-5, third annealing) after the above annealing (S7-3, second annealing), and the annealing conditions for this case are as described above.

In addition, in the production method according to the invention, in terms of softening, it is possible to conduct the above annealing process in combination with cold-rolling more than three times, and, even in this case, the process needs to be carried out within the above production conditions.

The carbon steel sheet according to the first embodiment of the invention can be described in the following manner, that is, a carbon steel sheet which includes, by mass %, C: more than or equal to 0.20% and less than or equal to 0.45%, Si: more than or equal to 0.05% and less than or equal to 0.8%, Mn: more than or equal to 0.85% and less than or equal to 2.0%, P: more than or equal to 0.001% and less than or equal to 0.04%, S: more than or equal to 0.0001% and less than or equal to 0.006%, Al: more than or equal to 0.01% and less than or equal to 0.1%, Ti: more than or equal to 0.005% and less than or equal to 0.3%, B: more than or equal to 0.0005% and less than or equal to 0.01% and N: more than or equal to 0.001% and less than or equal to 0.01% with a balance including Fe and inevitable impurities; has a value represented by 3C+Mn+0.5Si+Cr+Ni+Mo+Cu of 2.0 or more and surface hardness of the steel sheet of less than or equal to 77 on the Rockwell B Scale (HRB); has an average content of nitrogen (N) in a zone from the surface to a depth of 100 μm of 100 ppm or less; is used in a weak carburization atmosphere with a carbon potential (Cp) of 0.6 or less; and has excellent carburization properties. Here, C, Mn, Si, Cr, N, Mo and Cu represent the content of each element (% by mass) and are zero when the corresponding elements are not included.

The above carbon steel sheet may further include, by mass %, one or more components selected from Cr: 0.01% to 2.0%, Ni: 0.01% to 1.0%, Cu: 0.005% to 0.5% and Mo: 0.01% to 1.0%; and has a value represented by 3C+Mn+0.5Si+Cr+Ni+Mo+Cu which is greater than or equal to 2.0.

The above carbon steel sheet may further include, by mass %, one kind or two or more kinds of Nb: from 0.01% to 0.5%, V: from 0.01% to 0.5%, Ta: from 0.01% to 0.5% and W: from 0.01% to 0.5%.

The above carbon steel sheet may further include, by mass %, one kind or two or more kinds of Sn: from 0.003% to 0.03%, Sb: from 0.003% to 0.03%, and As: from 0.003% to 0.03%.

When hot-rolling a slab including the above components, a carbon steel sheet having excellent carburization properties may be produced by conducting heating at less than or equal to 1200° C.; having a final rolling temperature of hot-rolling of 800° C. to 940° C.; after completion of the final rolling, conducting cooling at a cooling rate of 20° C./second or more to 650° C.; subsequently, conducting cooling at a cooling rate of 20° C./second or less; conducting coiling at a coiling temperature of 400° C. to 650° C.; then conducting pickling; and then conducting annealing for more than or equal to 10 hours at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of 400° C. or more.

It is also possible to conduct the above annealing after conducting cold-rolling with a rolling ratio of 5% to 60% after the above pickling.

It is also possible to conduct another annealing at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of up to 400° C. and of less than or equal to −40° C. at a temperature of 400° C. or more after conducting cold-rolling with a rolling ratio of 5% to 60% after the above annealing.

It is also possible to conduct another annealing at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of up to 400° C. and of less than or equal to −40° C. at a temperature of 400° C. or more after conducting cold-rolling with a rolling ratio of 5% to 60% after the above second annealing.

With respect to annealing conducted on the above hot-rolled steel sheet or cold-rolled steel sheet, it is possible to conduct annealing in an atmosphere having a hydrogen content of 95% or more and at an annealing temperature in a range from “Ac1” to “Ac1+50° C.” and to conduct slow cooling at a cooling rate of 5° C./hour or less to “Ac1−30° C.” after the annealing.

Examples

The invention will be described based on examples.

Steel specimens obtained by casting steel including components shown in Tables 1 to 6 into 50 kg steel ingots by vacuum melting were hot-rolled under the conditions described in Tables 7 to 12. Heating for hot-rolling was conducted in the air atmosphere, and the thickness of hot-rolled steel sheets was 3 mm in the case of conducting no cold-rolling. In the case of conducting cold-rolling, the thickness of the hot-rolled steel sheets were controlled so that the cold-rolled steel sheets will become 3 mm. The hot-rolled steel sheets were pickled by hydrochloric acid and then subjected to annealing or cold-rolling so as to produce 3 mm-thick steel sheets for evaluation. The details on the production conditions and evaluation results are shown in Tables 7 to 12. After that, under the conditions described in Tables 7 to 12, a single annealing case, a cold-rolling and annealing case, a case in which a first annealing was followed by cold-rolling and then annealing again (annealing twice), and those repetition case (annealing three times) were carried out as shown in Tables 7 to 12 according to each treatment condition. With respect to the annealing atmosphere, the inside of a furnace was first substituted with nitrogen at room temperature, and then hydrogen was introduced until a predetermined amount of hydrogen was attained, and then the temperature was raised. In addition, dew points were measured using a dew-point meter with a thin film aluminum oxide moisture sensor.

Surface hardness of the obtained steel sheets were measured on the Rockwell B Scale (HRB), and the average amount of N in the surface layer was obtained by analyzing the content of nitrogen (N) in shavings taken by planing the surface layer portion of the steel sheet at a depth of 100 μm before carburization. Then, specimens which had been subjected to a profile forming process were carburized, and whether abnormal layers were present on the surface was investigated.

Meanwhile, the carburization treatment was conducted by the gas carburization method, and carbon potentials were measured by the CO₂ amount controlling method using an infrared gas analyzer.

The numeric parts in the “No.” column in Tables 7 to 12 are equivalent to the “No.” in Tables 1 to 6, therefore it is possible to distinguish materials with which components have been subjected to which conditions.

As shown in Tables 7 to 12, in conditions departing from the conditions of the invention (underlined) and in the comparative steel, lack of product hardness, cracks during the profiling forming process or abnormal layers in the surface layer portion during carburization were observed, which clarified the effects of the invention.

[Table 1]

[Table 2]

[Table 3]

[Table 4]

[Table 5]

[Table 6]

[Table 7]

[Table 8]

[Table 9]

[Table 10]

[Table 11]

[Table 12]

In general, since workability is degraded with an increase in surface hardness, it is preferable to maintain the surface hardness of steel sheets before a carburization treatment at less than or equal to a certain value in terms of securing product workability. The surface hardness HRB (Rockwell B Scale) of the steel sheets produced according to the conditions of the invention were all less than or equal to HRB 77, and it was confirmed from the results of the profile forming test (Tables 7 to 12) that, if the HRB is less than or equal to HRB 77, no cracks occur. That is, it was confirmed that the steel sheet according to the invention was excellent in terms of workability.

In addition, from the results shown in Tables 7 to 12, it was confirmed that the steel sheets according to the invention show sufficient performance even at a low carbon potential (Cp≦0.6), thereby being excellent in terms of not only carburization properties but also workability.

From the evaluation results of carburization properties, it was confirmed that none of the steel sheets produced according to the conditions of the invention included abnormal layers. That is, it was confirmed that the steel sheet according to the invention was excellent in terms of carburization properties.

INDUSTRIAL APPLICABILITY

As described above, according to the invention, it is possible to obtain a steel sheet which has excellent workability and can secure hardenability at the surface layer portion during carburization and a production method thereof. Since this steel sheet can be applied not only to automobile parts or a variety of industrial machine parts, but also to a wide range of tools or blades, the steel has a broad range of applications and can be used throughout many industries, therefore it is needless to say that this steel sheet is highly valuable in an industrial sense.

TABLE 1 Chemical composition (%) No C Si Mn P S Al Ti B N Cr Ni Cu Mo 1 0.24 0.12 1.26 0.023 0.0022 0.022 0.015 0.0035 0.0024 2 0.28 0.2 1.08 0.035 0.0055 0.032 0.022 0.0023 0.0035 3 0.35 0.24 0.87 0.018 0.0032 0.034 0.023 0.0039 0.0028 4 0.34 0.25 0.85 0.025 0.0022 0.057 0.014 0.0017 0.0033 0.15 5 0.22 0.33 1.13 0.013 0.0041 0.044 0.034 0.0033 0.0022 0.15 0.02 Chemical composition (%) K value Ac1 No Nb V Ta W Sn Sb As (K′ value) (° C.) Note 1 2.04 713 Invention steel 2 2.02 717 Invention steel 3 2.04 721 Invention steel 4 (2.15) 724 Invention steel 5 (2.11) 720 Invention steel

TABLE 2 Chemical composition (%) No C Si Mn P S Al Ti B N Cr Ni Cu Mo 6 0.21 0.05 1.22 0.009 0.0013 0.023 0.012 0.0019 0.0029 0.03 0.5 7 0.24 0.45 1.34 0.015 0.0025 0.046 0.023 0.0036 0.0035 0.12 0.06 0.08 8 0.22 0.22 1.25 0.022 0.0015 0.032 0.021 0.0028 0.0036 0.21 0.015 0.01 9 0.28 0.22 1.44 0.018 0.0054 0.026 0.028 0.0028 0.0019 10 0.33 0.42 0.88 0.027 0.005 0.033 0.033 0.0033 0.0032 Chemical composition (%) K value Ac1 No Nb V Ta W Sn Sb As (K′ value) (° C.) Note 6 (2.41) 703 Invention steel 7 (2.49) 720 Invention steel 8 (2.25) 719 Invention steel 9 0.03 2.39 719 Invention steel 10 0.21 2.08 726 Invention steel

TABLE 3 Chemical composition (%) No C Si Mn P S Al Ti B N Cr Ni Cu Mo 11 0.23 0.28 1.25 0.027 0.0023 0.031 0.015 0.0023 0.0037 12 0.29 0.23 1.11 0.017 0.0044 0.028 0.033 0.0028 0.0022 13 0.31 0.23 0.96 0.025 0.0033 0.045 0.027 0.0042 0.0027 14 0.22 0.42 1.82 0.033 0.0048 0.019 0.027 0.0028 0.0031 15 0.34 0.5 1.55 0.031 0.0021 0.033 0.049 0.0038 0.0028 Chemical composition (%) K value Ac1 No Nb V Ta W Sn Sb As (K′ value) (° C.) Note 11 0.28 2.08 719 Invention steel 12 0.08 2.10 718 Invention steel 13 0.015 0.3 2.01 719 Invention steel 14 0.03 0.28 2.69 716 Invention steel 15 0.023 0.08 2.82 721 Invention steel

TABLE 4 Chemical composition (%) No C Si Mn P S Al Ti B N Cr Ni Cu Mo 16 0.44 0.23 0.87 0.029 0.0025 0.048 0.029 0.0042 0.0025 17 0.22 0.21 1.18 0.023 0.0046 0.048 0.022 0.0048 0.0034 0.05 0.05 18 0.24 0.77 1.11 0.022 0.0028 0.076 0.028 0.0013 0.0036 0.35 19 0.29 0.13 1.05 0.008 0.0018 0.056 0.019 0.0017 0.0023 0.12 0.06 20 0.35 0.08 1.28 0.006 0.002 0.034 0.033 0.0022 0.0045 0.08 0.04 Chemical composition (%) K value Ac1 No Nb V Ta W Sn Sb As (K′ value) (° C.) Note 16 0.08 0.02 0.012 2.31 723 Invention steel 17 0.03 (2.05) 717 Invention steel 18 0.012 (2.57) 739 Invention steel 19 0.02 0.013 (2.17) 717 Invention steel 20 0.015 (2.45) 710 Invention steel

TABLE 5 Chemical composition (%) No C Si Mn P S Al Ti B N Cr Ni Cu Mo 21 0.44 0.23 1.47 0.012 0.0034 0.029 0.041 0.0016 0.0039 0.014 0.02 0.12 22 0.28 0.35 1.82 0.013 0.0029 0.041 0.015 0.0027 0.0022 0.08 23 0.33 0.18 1.56 0.026 0.0043 0.019 0.06 0.0037 0.0037 0.023 0.015 0.011 24 0.23 0.15 0.94 0.029 0.0038 0.09 0.21 0.0051 0.0018 0.45 0.73 0.021 25 0.34 0.21 0.87 0.037 0.0029 0.017 0.011 0.0044 0.0015 0.34 0.035 0.025 26 0.28 0.24 0.87 0.026 0.0039 0.047 0.023 0.0023 0.0024 0.12 0.024 0.125 27 0.23 0.15 1.38 0.014 0.0022 0.036 0.022 0.0036 0.0033 28 0.29 0.35 1.28 0.027 0.0027 0.028 0.024 0.0029 0.0025 0.12 0.06 0.11 29 0.35 0.34 1.34 0.019 0.0015 0.042 0.029 0.0026 0.0024 0.22 0.12 0.23 Chemical composition (%) K value Ac1 No Nb V Ta W Sn Sb As (K′ value) (° C.) Note 21 0.04 0.11 (3.06) 714 Invention steel 22 (2.84) 714 Invention steel 23 0.016 (2.69) 712 Invention steel 24 0.011 (2.89) 719 Invention steel 25 0.025 0.023 (2.37) 727 Invention steel 26 (2.08) 721 Invention steel 27 0.012 2.15 713 Invention steel 28 0.015 (2.56) 722 Invention steel 29 0.035 0.008 0.005 (3.13) 720 Invention steel

TABLE 6 Chemical composition (%) No C Si Mn P S Al Ti B N Cr Ni Cu Mo 30 0.23 0.1  0.59 0.025 0.0045 0.033 0.015 0.0022 0.0029 0.2 0.014 0.02 31 0.28 0.09 0.64 0.024 0.0033 0.043 0.021 0.0029 0.0032 0.23 0.014 0.02 32 0.35 0.07 0.52 0.022 0.0023 0.042 0.018 0.0022 0.0026 0.09 0.033 33 0.43 0.05 0.47 0.019 0.0034 0.029 0.017 0.0033 0.0035 0.09 0.04 34 0.24 0.83 1.25 0.025 0.0045 0.033 0.015 0.0025 0.0034 0.45 0.25 0.02 35 0.29 0.35 2.31 0.024 0.0057 0.037 0.028 0.0039 0.0033 0.23 0.014 36 0.44 0.19 2.13 0.019 0.0033 0.028 0.041 0.0018 0.0037 0.09 0.04 37 0.22 0.21 1.23 0.025 0.0034 0.035 0.022 0.0003 0.0035 0.21 0.014 Chemical composition (%) K value Ac1 No Nb V Ta W Sn Sb As (K′ value) (° C.) Note 30 (1.54) 723 Comparative Example 31 (1.77) 722 Comparative Example 32 (1.70) 721 Comparative Example 33 0.015 (1.92) 721 Comparative Example 34 (3.09) 737 Comparative Example 35 0.013 (3.60) 712 Comparative Example 36 0.015 (3.68) 707 Comparative Example 37 (2.22) 719 Comparative Example

TABLE 7 Cold rolling and annealing processes Hot rolling conditions (First) annealing Cooling Cooling Dew Dew Cooling Heating Reten- Final rate rate Coiling Cold point point at rate when temper- tion temper- to before temper- rolling thru 400° C. or annealing ature time ature 650° C. coiling ature ratio Atmo- 400° C. higher Annealing to Ac1 or No (° C.) (min) (° C.) (° C./s) (° C./s) (° C.) (%) sphere (° C.) (° C.) condition higher 1A 1150 30 860 40 15 620 — Hydrogen −30 −45 700° C. × — 100% 36 h 1B 1150 30 860 40 15 620 — Hydrogen −35 −50 700° C. × — 100% 36 h 1C 1150 30 860 40 15 620 — Hydrogen −30 −60 700° C. × — 100% 24 h 2A 1180 30 900 35 20 630 — Hydrogen −40 −60 710° C. × — 95% + 36 h nitrogen 5% 2B 1180 30 900 35 20 630 — Hydrogen −45 −60 640° C. × — 95% + 10 h nitrogen 5% 2C 1180 30 900 35 20 630 — Hydrogen −10 −40 710° C. × — 95% + 36 h nitrogen 5% 3A 1200 35 860 40 10 600 — Hydrogen −30 −60 710° C. × — 100% 48 h 3B 1280 40 900 30 15 620 — Hydrogen −35 −55 710° C. × — 100% 48 h 3C 1200 60 860 40 10 580 — Hydrogen −35 −60 660° C. × — 100% 6 h 4A 1150 40 880 25 10 600 — Hydrogen −40 −65 710° C. × — 95% + 48 h nitrogen 5% 4B 1150 40 880 25 10 600 — Hydrogen −30 −35 710° C. × — 95% + 48 h nitrogen 5% 5A 1150 40 880 20 5 580 — Hydrogen −30 −50 700° C. × — 100% 48 h 5B 1150 40 880 20 5 580 30 Hydrogen −25 −55 700° C. × — 100% 36 h Cold rolling and annealing processes (Second) annealing (Third) annealing Dew Dew Cooling Dew Dew Cold point point at rate when Cold point point at rolling thru 400° C. or annealing rolling thru 400° C. or ratio Atmo- 400° C. higher Annealing to Ac1 or ratio Atmo- 400° C. higher No No. (%) sphere (° C.) (° C.) condition higher (%) sphere (° C.) (° C.) 1A 1A — — — — — — — — — — 1B 1B — — — — — — — — — — 1C 1C 20 Hydrogen −35 −55 725° C. × 1° C./hr — — — — 100% 10 h 2A 2A — — — — — — — — — — 2B 2B — — — — — — — — — — 2C 2C — — — — — — — — — — 3A 3A — — — — — — — — — — 3B 3B — — — — — — — — — — 3C 3C — — — — — — — — — — 4A 4A — — — — — — — — — — 4B 4B — — — — — — — — — — 5A 5A — — — — — — — — — — 5B 5B — — — — — — — — — — Cold rolling and annealing processes (Third) annealing Cooling Sheet product characteristics rate when Amount of N in Product crack Carburized annealing sheet products hard- during material Annealing to Ac1 or thru a depth of ness profile abnormal No condition higher 100 μm (ppm) (HRB) forming Cp layers Note 1A — — 51 71 — 0.3 Not Present present invention 1B — — 43 71 — 0.38 Not Present present invention 1C — — 74 68 — 0.45 Not Present present invention 2A — — 67 74 — 0.3 Not Present present invention 2B — — 58 82 Crack 0.3 Not Comparative occurred present Example 2C — — 104  73 — 0.35 Present Comparative Example 3A — — 39 74 — 0.45 Not Present present invention 3B — — 120  75 — 0.45 Present Comparative Example 3C — — 36 83 Crack 0.45 Not Comparative occurred present Example 4A — — 84 75 — 0.45 Not Present present invention 4B — — 144  74 — 0.45 Present Comparative Example 5A — — 35 75 — 0.3 Not Present present invention 5B — — 33 72 — 0.3 Not Present present invention

TABLE 8 Cold rolling and annealing processes Hot rolling conditions (First) annealing Cooling Cooling Dew Dew Cooling Heating Reten- Final rate rate Coiling Cold point point at rate when temper- tion temper- to before temper- rolling thru 400° C. or annealing ature time ature 650° C. coiling ature ratio Atmo- 400° C. higher Annealing to Ac1 or No (° C.) (min) (° C.) (° C./s) (° C./s) (° C.) (%) sphere (° C.) (° C.) condition higher  6A 1180 40 880 35 20 550 — Hydrogen −40 −60 690° C. × — 100% 36 h  6B 1180 40 880 35 20 550 — Hydrogen −30 −60 690° C. × — 90% + 48 h Nitrogen 10%  7A 1180 30 840 40 15 620 — Hydrogen −30 −50 700° C. × — 100% 36 h  7B 1180 30 840 40 15 620 — Hydrogen −30 −50 700° C. × — 100% 36 h  8A 1180 30 840 40 15 630 — Hydrogen −25 −45 690° C. × — 100% 24 h  8B 1260 30 850 30 20 600 — Hydrogen −35 −60 690° C. × — 100% 48 h  9A 1180 30 830 35 20 580 — Hydrogen −50 −60 710° C. × — 100% 36 h  9B 1200 40 860 30 15 640 — Hydrogen −35 −60 735° C. × 10° C./hr 100% 12 h  9C 1200 60 860 30 15 640 — Hydrogen −40 −60 690° C. × — 90% + 48 h Nitrogen 10%  9D 1200 90 840 30 15 620 — Hydrogen −35 −60 690° C. × — 95% + 36 h nitrogen 5% 10A 1100 60 840 40 20 580 — Hydrogen −30 −55 710° C. × — 100% 36 h 10B 1100 60 840 40 20 580 — Hydrogen −15 −45 710° C. × — 100% 48 h Cold rolling and annealing processes (Second) annealing (Third) annealing Dew Dew Cooling Dew Dew Cold point point at rate when Cold point point at rolling thru 400° C. or annealing rolling thru 400° C. or ratio Atmo- 400° C. higher Annealing to Ac1 or ratio Atmo- 400° C. higher No No. (%) sphere (° C.) (° C.) condition higher (%) sphere (° C.) (° C.)  6A 6A — — — — — — — — — —  6B 6B — — — — — — — — — —  7A 7A 15 Hydrogen −40 −60 700° C. × — — — — — 100% 36 h  7B 7B 15 Hydrogen −20 −35 700° C. × — — — — — 100% 36 h  8A 8A 20 Hydrogen −40 −60 690° C. × — — — — — 100% 24 h  8B 8B — — — — — — — — — —  9A 9A — — — — — — — — — —  9B 9B — — — — — — — — — —  9C 9C — — — — — — — — — —  9D 9D — — — — — — — — — — 10A 10A — — — — — — — — — — 10B 10B — — — — — — — — — — Cold rolling and annealing processes (Third) annealing Cooling Sheet product characteristics rate when Amount of N in Product crack Carburized annealing sheet products hard- during material Annealing to Ac1 or thru a depth of ness profile abnormal No condition higher 100 μm (ppm) (HRB) forming Cp layers Note 6A — — 48 75 — 0.3 Not Present present invention 6B — — 108  74 — 0.3 Present Comparative Example 7A — — 66 74 — 0.3 Not Present present invention 7B — — 66 74 — 0.3 Present Comparative Example 8A — — 89 73 — 0.3 Not Present present invention 8B — — 143  74 — 0.3 Present Comparative Example 9A — — 29 74 — 0.3 Not Present present invention 9B — — 36 80 Crack 0.3 Not Comparative occurred present Example 9C — — 110  74 — 0.3 Present Comparative Example 9D — — 103  75 — 0.3 Present Comparative Example 10A — — 47 74 — 0.45 Not Present present invention 10B — — 58 72 — 0.45 Present Comparative Example

TABLE 9 Cold rolling and annealing processes Hot rolling conditions (First) annealing Cooling Cooling Dew Dew Cooling Heating Reten- Final rate rate Coiling Cold point point at rate when temper- tion temper- to before temper- rolling thru 400° C. or annealing ature time ature 650° C. coiling ature ratio Atmo- 400° C. higher Annealing to Ac1 or No (° C.) (min) (° C.) (° C./s) (° C./s) (° C.) (%) sphere (° C.) (° C.) condition higher 11A 1100 50 880 25 15 600 — Hydrogen −35 −50 690° C. × — 100% 36 h 11B 1100 50 880 25 15 600 — Hydrogen 5% + −30 −45 690° C. × — Nitrogen 95% 24 h 11C 1100 50 880 25 15 600 — Hydrogen −40 −65 650° C. × — 100% 12 h 11D 1100 150 880 25 15 600 — Hydrogen −40 −65 690° C. × — 100% 36 h 12A 1100 45 900 30 10 570 — Hydrogen −40 −60 710° C. × — 100% 36 h 12B 1100 45 900 30 10 570 — Hydrogen −40 −65 630° C. × — 100% 10 h 12C 1100 45 900 30 10 570 — Hydrogen −35 −60 730° C. × 7° C./hr 100% 10 h 12D 1100 120 880 25 10 600 — Hydrogen −35 −45 890° C. × — 100% 36 h 13A 1200 30 820 30 10 540 — Hydrogen −45 −60 710° C. × — 100% 36 h 13B 1200 30 820 30 10 540 30 Hydrogen −40 −60 710° C. × — 100% 36 h 14A 1200 30 800 30 8 590 — Hydrogen −35 −55 690° C. × — 100% 24 h 14B 1200 30 800 30 8 590 — Hydrogen −35 −55 690° C. × — 100% 24 h 15A 1200 30 840 35 13 600 — Hydrogen −30 −50 710° C. × — 100% 36 h 15B 1200 30 840 35 13 600 20 Hydrogen −30 −50 690° C. × — 100% 12 h 15C 1200 30 840 35 13 600 20 Hydrogen −30 −50 690° C. × — 100% 12 h Cold rolling and annealing processes (Second) annealing (Third) annealing Dew Dew Cooling Dew Dew Cold point point at rate when Cold point point at rolling thru 400° C. or annealing rolling thru 400° C. or ratio Atmo- 400° C. higher Annealing to Ac1 or ratio Atmo- 400° C. higher No No. (%) sphere (° C.) (° C.) condition higher (%) sphere (° C.) (° C.) 11A 11A — — — — — — — — — — 11B 11B — — — — — — — — — — 11C 11C — — — — — — — — — 11D 11C — — — — — — — — — 12A 12A — — — — — — — — — — 12B 12B — — — — — — — — — — 12C 12C — — — — — — — — — — 12D 12B — — — — — — — — — — 13A 13A — — — — — — — — — — 13B 13B — — — — — — — — — — 14A 14A 25 Hydrogen −35 −55 690° C. × — 30 Hydrogen −35 −55 100% 36 h 100% 14B 14B 25 Hydrogen −35 −55 690° C. × — 30 Hydrogen −35 −55 100% 36 h 80% + Nitrogen 20% 15A 15A 20 Hydrogen −20 −50 710° C. × — 25 Hydrogen −45 −60 100% 24 h 100% 15B 15B 25 −20 −50 680° C. × — — — — — 6 h 15C 25 Hydrogen −30 −60 690° C. × — — — — — 92% + 36 h Nitrogen Cold rolling and annealing processes (Third) annealing Cooling Sheet product characteristics rate when Amount of N in Product crack Carburized annealing sheet products hard- during material Annealing to Ac1 or thru a depth of ness profile abnormal No condition higher 100 μm (ppm) (HRB) forming Cp layers Note 11A — — 52 73 — 0.45 Not Present present invention 11B — — 323  75 — 0.45 Present Comparative Example 11C — — 45 83 Crack 0.45 Not Comparative occurred present Example 11D — — 125  74 — 0.3 Present Comparative Example 12A — — 41 75 — 0.33 Not Present present invention 12B — — 34 81 Crack 0.33 Not Comparative occurred present Example 12C — — 46 79 Crack 0.33 Not Comparative occurred present Example 12D — — 108  76 — 0.33 Present Comparative Example 13A — — 45 77 — 0.45 Not Present present invention 13B — — 40 73 — 0.45 Not Present present invention 14A 680° C. × — 61 69 — 0.6 Not Present 12 h present invention 14B 680° C. × — 129  69 — 0.3 Present Comparative 12 h Example 15A 680° C. × — 67 71 — 0.35 Not Present 12 h present invention 15B — — 60 79 Crack 0.35 Not Comparative occurred present Example 15C — — 108  72 — 0.35 Present Comparative Example

TABLE 10 Cold rolling and annealing processes Hot rolling conditions (First) annealing Cooling Cooling Dew Dew Cooling Heating Reten- Final rate rate Coiling Cold point point at rate when temper- tion temper- to before temper- rolling thru 400° C. or annealing ature time ature 650° C. coiling ature ratio Atmo- 400° C. higher Annealing to Ac1 or No (° C.) (min) (° C.) (° C./s) (° C./s) (° C.) (%) sphere (° C.) (° C.) condition higher 16A 1180 40 860 25 20 550 — Hydrogen −35 −60 710° C. × — 100% 36 h 16B 1180 40 860 25 20 550 — Hydrogen −20 −35 700° C. × — 100% 48 h 16C 1180 40 860 25 20 550 — Hydrogen −35 −60 700° C. × — 100% 12 h 17A 1180 50 860 30 20 500 — Hydrogen −30 −50 690° C. × — 100% 36 h 17B 1180 50 860 30 20 500 20 Hydrogen −30 −55 690° C. × — 100% 48 h 17C 1220 80 840 40 15 600 — Hydrogen −25 −55 690° C. × — 100% 24 h 17D 1180 50 860 30 20 500 — Hydrogen −25 −55 690° C. × — 100% 24 h 18A 1180 40 880 20 10 500 — Hydrogen −20 −45 710° C. × — 100% 48 h 18B 1180 40 880 20 10 500 50 Hydrogen −25 −40 710° C. × — 100% 36 h 18C 1180 40 880 20 10 500 50 Hydrogen −25 −40 710° C. × — 100% 36 h 19A 1180 40 880 40 15 560 — Hydrogen −30 −45 710° C. × — 100% 36 h 19B 1180 40 880 40 15 560 40 Hydrogen −30 −40 730° C. × 2° C./hr 100% 10 h 20A 1170 30 850 35 10 480 — Hydrogen −40 −60 700° C. × — 100% 12 h 20B 1170 30 850 35 10 480 — Hydrogen −40 −60 700° C. × — 100% 12 h Cold rolling and annealing processes (Second) annealing (Third) annealing Dew Dew Cooling Dew Dew Cold point point at rate when Cold point point at rolling thru 400° C. or annealing rolling thru 400° C. or ratio Atmo- 400° C. higher Annealing to Ac1 or ratio Atmo- 400° C. higher No No. (%) sphere (° C.) (° C.) condition higher (%) sphere (° C.) (° C.) 16A 16A — — — — — — — — — — 16B 16B — — — — — — — — — — 16C 16C 15 Hydrogen −25 −60 700° C. × —  5 Hydrogen −35 −55 100% 36 h 100% 17A 17A — — — — — — — — — — 17B 17B — — — — — — — — — — 17C 17C 20 Hydrogen −40 −60 700° C. × — — — — — 100% 24 h 17D 17D 20 Hydrogen −15 −45 700° C. × — — — — — 100% 36 h 18A 18A 25 Hydrogen −35 −60 700° C. × — 30 Hydrogen −40 −65 100% 36 h 100% 18B 18B 10 Hydrogen −20 −50 700° C. × — — — — — 100% 24 h 18C 18C 10 Hydrogen −10 −40 700° C. × — — — — — 100% 36 h 19A 19A — — — — — — — — — — 19B 19B — — — — — — — — — — 20A 20A 30 Hydrogen −30 −60 725° C. × 2° C./hr — — — — 100% 12 h 20B 20B 30 −30 −60 690° C. × — — — — — 36 h Cold rolling and annealing processes (Third) annealing Cooling Sheet product characteristics rate when Amount of N in Product crack Carburized annealing sheet products hard- during material Annealing to Ac1 or thru a depth of ness profile abnormal No condition higher 100 μm (ppm) (HRB) forming Cp layers Note 16A — — 43 74 — 0.6 Not Present present invention 16B — — 49 73 — 0.48 Present Comparative Example 16C 730° C. × 1° C./hr 51 71 — 0.48 Not Present 12 h present invention 17A — — 51 71 — 0.3 Not Present present invention 17B — — 58 69 — 0.3 Not Present present invention 17C — — 135  71 — 0.3 Present Comparative Example 17D — — 65 70 — 0.3 Present Comparative Example 18A 690° C. × — 78 74 — 0.3 Not Present 12 h present invention 18B — — 59 73 — 0.3 Not Present present invention 18C — — 64 72 — 0.3 Present Comparative Example 19A — — 73 74 — 0.45 Not Present present invention 19B — — 92 73 — 0.45 Not Present present invention 20A — — 63 71 — 0.45 Not Present present invention 20B — — 111  73 — 0.4 Present Comparative Example

TABLE 11 Cold rolling and annealing processes Hot rolling conditions (First) annealing Cooling Cooling Dew Dew Cooling Heating Reten- Final rate rate Coiling Cold point point at rate when temper- tion temper- to before temper- rolling thru 400° C. or annealing ature time ature 650° C. coiling ature ratio Atmo- 400° C. higher Annealing to Ac1 or No (° C.) (min) (° C.) (° C./s) (° C./s) (° C.) (%) sphere (° C.) (° C.) condition higher 21A 1170 30 890 45 8 520 — Hydrogen −35 −55 710° C. × — 100% 36 h 21B 1170 30 890 45 8 520 30 Hydrogen −30 −65 730° C. × 1° C./hr 100% 10 h 21C 1170 30 890 45 8 520 — Hydrogen −40 −65 700° C. × — 100% 36 h 21D 1180 40 880 35 10 580 — Hydrogen −40 −65 700° C. × — 100% 36 h 22A 1120 30 910 50 10 550 — Hydrogen −35 −60 710° C. × — 100% 36 h 22B 1120 30 910 50 10 550 — Hydrogen −15 −35 710° C. × — 100% 36 h 23 1120 50 910 35 15 580 — Hydrogen −30 −50 700° C. × — 100% 36 h 24 1100 60 830 20 15 600 25 Hydrogen −30 −45 710° C. × — 100% 24 h 25 1100 60 880 25 20 600 — Hydrogen −40 −60 710° C. × — 100% 36 h 26 1100 60 860 35 15 540 — Hydrogen −35 −55 710° C. × — 100% 36 h 27 1180 40 870 30 10 600 — Hydrogen −30 −60 690° C. × — 95% + 72 h nitrogen 5% 28 1180 40 870 25 20 600 — Hydrogen −25 −50 690° C. × — 95% + 72 h nitrogen 5% 29 1180 40 860 30 15 600 30 Hydrogen −40 −55 690° C. × — 95% + 36 h nitrogen 5% Cold rolling and annealing processes (Second) annealing (Third) annealing Dew Dew Cooling Dew Dew Cold point point at rate when Cold point point at rolling thru 400° C. or annealing rolling thru 400° C. or ratio Atmo- 400° C. higher Annealing to Ac1 or ratio Atmo- 400° C. higher No No. (%) sphere (° C.) (° C.) condition higher (%) sphere (° C.) (° C.) 21A 21A 25 Hydrogen −35 −50 710° C. × — — — — — 100% 48 h 21B 21B — — — — — — — — — — 21C 21C 30 Hydrogen −30 −55 730° C. × 10° C./hr — — — — 100% 10 h 21D 21D 10 Hydrogen −30 −55 700° C. ×  5 Hydrogen −35 −55 100% 12 h 100% 22A 22A 15 Hydrogen −25 −60 710° C. × — 15 Hydrogen −35 −55 100% 36 h 100% 22B 22B 15 Hydrogen −40 −65 710° C. × — 15 Hydrogen −40 −60 100% 36 h 100% 23 23 30 Hydrogen −45 −60 700° C. × — 20 Hydrogen −40 −55 100% 24 h 100% 24 24 — — — — — — — — — — 25 25 — — — — — — — — — — 26 26 — — — — — — — — — — 27 27 — — — — — — — — — — 28 28 — — — — — — — — — — 29 29  5 Hydrogen −40 −55 690° C. × — — — — — 95% + 12 h nitrogen 5% Cold rolling and annealing processes (Third) annealing Cooling Sheet product characteristics rate when Amount of N in Product crack Carburized annealing sheet products hard- during material Annealing to Ac1 or thru a depth of ness profile abnormal No condition higher 100 μm (ppm) (HRB) forming Cp layers Note 21A — — 76 72 — 0.6 Not Present present invention 21B — — 55 72 — 0.6 Not Present present invention 21C — — 64 79 Crack 0.6 Not Comparative occurred present Example 21D 730° C. × 2° C./hr 64 69 — 0.6 Not Present 12 h present invention 22A 700° C. × — 48 68 — 0.3 Not Present 24 h present invention 22B 700° C. × — 116  68 — 0.3 Present Comparative 24 h Example 23 690° C. × — 72 70 — 0.45 Not Present 12 h present invention 24 — — 44 71 — 0.3 Not Present present invention 25 — — 32 74 — 0.45 Not Present present invention 26 — — 56 72 — 0.3 Not Present present invention 27 — — 61 73 — 0.3 Not Present present invention 28 — — 53 74 — 0.35 Not Present present invention 29 — — 48 74 — 0.4 Not Present present invention

TABLE 12 Cold rolling and annealing processes Hot rolling conditions (First) annealing Cooling Cooling Dew Dew Cooling Heating Reten- Final rate rate Coiling Cold point point at rate when temper- tion temper- to before temper- rolling thru 400° C. or annealing ature time ature 650° C. coiling ature ratio Atmo- 400° C. higher Annealing to Ac1 or No (° C.) (min) (° C.) (° C./s) (° C./s) (° C.) (%) sphere (° C.) (° C.) condition higher 30 1200 40 860 25 15 550 — Hydrogen −30 −45 710° C. × — 100% 24 h 31 1200 40 880 30 10 600 — Hydrogen −40 −55 710° C. × — 100% 36 h 32 1180 30 670 35 10 620 — Hydrogen −40 −60 710° C. × — 100% 36 h 33 1180 30 830 30 20 590 — Hydrogen −35 −55 710° C. × — 100% 36 h 34 1150 60 840 40 20 540 — Hydrogen −35 −50 710° C. × — 100% 24 h 35 1150 60 900 25 15 580 — Hydrogen −25 −45 710° C. × — 100% 36 h 36 1150 40 870 30 5 450 — Hydrogen −30 −55 700° C. × — 100% 36 h 37 1180 30 850 30 15 600 — Hydrogen −25 −40 690° C. × — 100% 24 h Cold rolling and annealing processes (Second) annealing (Third) annealing Dew Dew Cooling Dew Dew Cold point point at rate when Cold point point at rolling thru 400° C. or annealing rolling thru 400° C. or ratio Atmo- 400° C. higher Annealing to Ac1 or ratio Atmo- 400° C. higher No No. (%) sphere (° C.) (° C.) condition higher (%) sphere (° C.) (° C.) 30 30 — — — — — — — — — — 31 31 — — — — — — — — — — 32 32 — — — — — — — — — — 33 33 15 Hydrogen −35 −50 710° C. × — — — — — 100% 12 h 34 34 20 Hydrogen −40 −55 710° C. × — 25 Hydrogen −30 −45 100% 12 h 100% 35 35 15 Hydrogen −30 −50 710° C. × — 20 Hydrogen −35 −55 100% 24 h 100% 36 36 20 Hydrogen −30 −50 700° C. × — 15 Hydrogen −45 −55 100% 12 h 100% 37 37 20 Hydrogen −45 −60 690° C. × — — — — — 100% 24 h Cold rolling and annealing processes (Third) annealing Cooling Sheet product characteristics rate when Amount of N in Product crack Carburized annealing sheet products hard- during material Annealing to Ac1 or thru a depth of ness profile abnormal No condition higher 100 μm (ppm) (HRB) forming Cp layers Note 30 — — 39 73 — 0.3 Present Comparative Example 31 — — 46 72 — 0.3 Present Comparative Example 32 — — 39 71 — 0.45 Present Comparative Example 33 — — 48 69 — 0.45 Present Comparative Example 34 680° C. × — 51 80 Crack 0.3 Not Comparative 12 h occurred present Example 35 680° C. × — 49 81 Crack 0.45 Not Comparative 12 h occurred present Example 36 680° C. × — 53 82 Crack 0.6 Not Comparative 12 h occurred present Example 37 — — 70 69 — 0.3 Present Comparative Example 

1. A carbon steel sheet configured to be carburized in a carburization atmosphere with a carbon potential of 0.6 or less, comprising: C: 0.20% to 0.45% by mass, Si: 0.05% to 0.8% by mass, Mn: 0.85% to 2.0% by mass, P: 0.001% to 0.04% by mass, S: 0.0001% to 0.006% by mass, Al: 0.01% to 0.1% by mass, Ti: 0.005% to 0.3% by mass, B: 0.0005% to 0.01% by mass and N: 0.001% to 0.01% by mass with a balance including Fe and inevitable impurities, wherein K value that can be obtained from 3C+Mn+0.5Si is greater than or equal to 2.0; surface hardness is less than or equal to 77 on Rockwell B Scale; and an average content of N in a zone from a surface to a depth of 100 μm is less than or equal to 100 ppm.
 2. The carbon steel sheet according to claim 1, further comprising one or more components selected from Nb: 0.01% to 0.5% by mass, V: 0.01% to 0.5% by mass, Ta: 0.01% to 0.5% by mass, W: 0.01% to 0.5% by mass, Sn: 0.003% to 0.03% by mass, Sb: 0.003% to 0.03% by mass, and As: 0.003% to 0.03% by mass.
 3. A method for producing the carbon steel sheet according to claim 1, comprising: heating a slab to less than or equal to 1200° C.; hot-rolling the slab at a final rolling temperature of 800° C. to 940° C. so as to obtain a steel sheet; cooling the steel sheet at a cooling rate of 20° C./second or more until a temperature of the steel sheet becomes less than or equal to 650° C., as a first cooling; cooling the steel sheet at a cooling rate of 20° C./second or less, as a second cooling subsequent to the first cooling; coiling the steel sheet at a temperature of 400° C. to 650° C.; pickling the steel sheet; and annealing the steel sheet for 10 hours or more at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C., as a first annealing.
 4. The method for producing the carbon steel sheet according to claim 3, wherein the first annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the first annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.
 5. The method for producing the carbon steel sheet according to claim 4, further comprising: cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the pickling, as a first cold-rolling.
 6. The method for producing the carbon steel sheet according to claim 5, further comprising: cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.
 7. The method for producing the carbon steel sheet according to claim 6, wherein the second annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.
 8. The method for producing the carbon steel sheet according to claim 7, further comprising: cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.
 9. The method for producing the carbon steel sheet according to claim 8, wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.
 10. The method for producing the carbon steel sheet according to claim 6, further comprising: cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.
 11. The method for producing the carbon steel sheet according to claim 10, wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.
 12. The method for producing the carbon steel sheet according to claim 4, further comprising: cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.
 13. The method for producing the carbon steel sheet according to claim 12, wherein the second annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.
 14. The method for producing the carbon steel sheet according to claim 13, further comprising: cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.
 15. The method for producing the carbon steel sheet according to claim 14, wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.
 16. The method for producing the carbon steel sheet according to claim 12, further comprising: cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.
 17. The method for producing the carbon steel sheet according to claim 16, wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.
 18. The method for producing the carbon steel sheet according to claim 3, further comprising: cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the pickling, as a first cold-rolling.
 19. The method for producing the carbon steel sheet according to claim 18, further comprising: cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.
 20. The method for producing the carbon steel sheet according to claim 19, wherein the second annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.
 21. The method for producing the carbon steel sheet according to claim 20, further comprising: cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.
 22. The method for producing the carbon steel sheet according to claim 21, wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.
 23. The method for producing the carbon steel sheet according to claim 19, further comprising: cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.
 24. The method for producing the carbon steel sheet according to claim 23, wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.
 25. The method for producing the carbon steel sheet according to claim 3, further comprising: cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.
 26. The method for producing the carbon steel sheet according to claim 25, wherein the second annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.
 27. The method for producing the carbon steel sheet according to claim 26, further comprising: cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.
 28. The method for producing the carbon steel sheet according to claim 27, wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.
 29. The method for producing the carbon steel sheet according to claim 25, further comprising: cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.
 30. The method for producing the carbon steel sheet according to claim 29, wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.
 31. A carbon steel sheet configured to be carburized in a carburization atmosphere with a carbon potential of 0.6 or less, comprising C: 0.20% to 0.45% by mass, Si: 0.05% to 0.8% by mass, Mn: 0.85% to 2.0% by mass, P: 0.001% to 0.04% by mass, S: 0.0001% to 0.006% by mass, Al: 0.01% to 0.1% by mass, Ti: 0.005% to 0.3% by mass, B: 0.0005% to 0.01% by mass and N: 0.001% to 0.01% by mass, and further comprising one or more components selected from: Cr: 0.01% to 2.0% by mass, Ni: 0.01% to 1.0% by mass, Cu: 0.005% to 0.5% by mass and Mo: 0.01% to 1.0% by mass with a balance including Fe and inevitable impurities, wherein K′ value that can be obtained from 3C+Mn+0.5Si+Cr+Ni+Mo+Cu is greater than or equal to 2.0; surface hardness is less than or equal to 77 on Rockwell B Scale; and an average content of N in a zone from a surface to a depth of 100 μM is less than or equal to 100 ppm.
 32. The carbon steel sheet according to claim 31, further comprising one or more components selected from: Nb: 0.01% to 0.5% by mass, V: 0.01% to 0.5% by mass, Ta: 0.01% to 0.5% by mass, W: 0.01% to 0.5% by mass, Sn: 0.003% to 0.03% by mass, Sb: 0.003% to 0.03% by mass, and As: 0.003% to 0.03% by mass.
 33. The method for producing the carbon steel sheet according to claim 31, comprising: heating a slab to less than or equal to 1200° C.; hot-rolling the slab at a final rolling temperature of 800° C. to 940° C. so as to obtain a steel sheet; cooling the steel sheet at a cooling rate of 20° C./second or more until a temperature of the steel sheet becomes less than or equal to 650° C., as a first cooling; cooling the steel sheet at a cooling rate of less than or equal to 20° C./second, as a second cooling subsequent to the first cooling; coiling the steel sheet at a temperature of 400° C. to 650° C.; pickling the steel sheet; and annealing the steel sheet for more than or equal to 10 hours at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C., as a first annealing. 